In many applications, data indicative of physical conditions are sensed at various locations around a network. In such applications, the data are often tagged with timing information. For example, in exploring for oil, an area may be surveyed using sensors (accelerometers) disposed in a grid. Seismic shocks are induced by triggers such as explosives or vibrator trucks, and in response the sensors provide data indicative of soil content and condition. Data from each sensor is time-stamped and sent to a central processor for analysis. Free-running clocks at the sensors may be used to time-stamp the data.